Application of solar power supply system in communication base stations

Currently, out of the world's 6.6 billion population, over 2 billion people do not have sufficient electricity supply, accounting for approximately one-third of the total population. Figure 1 shows the regions of the world with and without electricity.

The areas without sufficient power supply are mainly distributed in Africa, South America, Asia, and Southeast Asia. Like the Philippines and Indonesia, which have numerous islands, it is impossible to build a large-scale power grid in these small island areas. In some areas, the cost of building and maintaining large-scale power grids is too high, such as the remote northwest region of China, which is vast and sparsely populated. It is unreasonable from an economic perspective to introduce the power grid into every herder family.

In some places where major high-voltage transmission networks have been established, power supply is often unstable, and upgrades and renovations require huge financial budgets. Fortunately, many developing countries have abundant renewable energy sources such as solar or wind power, and using these renewable energy sources on a large scale in remote areas for power supply systems is more cost-effective than using large-scale high-voltage transmission grids. The power supply system in remote areas can be applied in situations where there is already an existing power grid, but separate power supply is more cost-effective than expanding the high-voltage transmission network. For example, using an independent power supply system along highways for signal indication, communication, and lighting can avoid expensive projects such as laying and maintaining underground cables. Regions with abundant global solar energy resources include Africa, South Asia, Southeast Asia, Australia, Central America, and China's Qinghai Tibet Plateau. Using solar power systems to supply electricity in these regions is an economic choice.

1. Selection of power supply system for communication base stations in remote areas

The power supply system in remote areas generally includes power generation equipment, energy storage equipment, energy conversion and management equipment. Power generation equipment includes diesel generators, photovoltaic arrays, wind turbines, or hydroelectric generators. Energy storage devices generally have battery packs or energy storage pools. Energy conversion and management equipment includes DC converters, inverters, and other devices.

Diesel generators are the energy source for many remote power supply systems. In order to achieve maximum fuel efficiency and reduce maintenance, the load rate needs to be maintained at 60% to 70% of the rated load capacity of the generator. The output power of wind turbines can reach 250W~500kW, but it is necessary to select an appropriate wind farm with stable wind speed. Although hydro generators have relatively low power generation costs, they need to be built on moderate and stable rivers. The power generation cost of hydro generators is relatively low, but the cost of generators is high.

The communication network requires base stations and other equipment to provide stable operation 24/7. Base station equipment is not only distributed in urban areas, but also in various environments such as deserts, islands, and mountaintops, with a wide coverage area and generally unmanned operation. It has high requirements for power reliability and lifespan. The photovoltaic cells of the solar power supply system directly convert solar energy into electrical energy, and provide the required -48V voltage for the base station through the series parallel connection of photovoltaic modules, achieving static energy conversion. Compared with generators with mechanical rotating parts, the maintenance workload is minimal. For base station loads less than 2kW, it is a suitable power supply system solution for remote areas, especially in the context of high global crude oil prices, where the cost advantage of photovoltaic power generation systems is becoming increasingly apparent.

2. Photovoltaic power supply system for communication base stations

The solar power supply system for communication base stations consists of photovoltaic modules, array brackets, junction boxes, charge and discharge controllers, battery packs, inverters, etc.

Components are generally made of monocrystalline silicon or polycrystalline silicon cells, with an output voltage of approximately 0.5V per cell. Typically, 72 solar cells are connected in series, so in order to obtain a voltage range of 43.2-56.4V, two components need to be used in series. Try to choose specifications with higher output for power levels, such as 165W, 170W, and 175W. A component specification that is too small leads to an increase in bracket design costs and footprint, while a component specification that is too large results in a lower yield of solar cells and relatively higher battery costs. Select the number of components in parallel based on load capacity and local solar energy resources.

Multiple photovoltaic modules are connected in parallel to form an array, supported by galvanized steel brackets to provide the modules with a certain tilt angle, while fixing the modules to resist wind blowing. For independent photovoltaic systems, in order to reduce battery usage and system costs, it is necessary to obtain maximum solar radiation in winter, which requires setting the tilt angle of the components to be 10 °~20 ° higher than the local latitude.

When it is rainy or nighttime, with no sunlight or weakened radiation, the battery pack continues to provide the energy needed by the load. The capacity of the battery pack is determined based on the load capacity, the number of days of self-sufficiency during continuous rainy days, and the depth of discharge.

In the past, the rich liquid lead-acid battery (OPzS) was a commonly used choice for photovoltaic power supply systems. This is because OPzS batteries use tubular positive electrodes to prevent the active material from falling off, and thick negative electrode plates to extend their service life. However, in recent years, more and more photovoltaic systems have shifted towards colloidal valve regulated sealed lead-acid batteries (OPzVs) with tubular positive electrode plates. The main reason for this shift is that valve regulated sealed lead-acid battery (VRLA) technology requires less maintenance.

Rich liquid batteries require regular water maintenance. If not maintained in a timely manner, the service life of the battery will be shortened, and transporting deionized distilled water to remote base stations requires higher costs. Under normal working conditions, VRLA batteries only release trace amounts of sulfuric acid and hydrogen gas, greatly reducing maintenance workload and eliminating the need for specialized equipment rooms and ventilation installations. The phenomenon of electrolyte stratification is a common cause of failure in many electrolyte rich batteries, which is usually eliminated by overcharging, typically requiring an additional overcharge of up to 15%. Colloidal batteries experience negligible electrolyte delamination during operation and therefore do not suffer from delamination related failures. Undercharging is a common cause of VRLA failure in remote power supply systems. This is due to the accumulation and growth of lead sulfate crystals in the active material of the battery caused by unstable photovoltaic energy sources during the rainy season. Studies have shown that the microporous separators used in colloidal batteries are less prone to dendrite penetration and have better characteristics in this regard. Compared with the charging recovery ability of 110%~115% for rich liquid batteries, the charging recovery of colloidal batteries is only 103%~105%. The improvement in charging efficiency is beneficial for saving photovoltaic energy.

The charge and discharge control adopts a multi-channel controller, and the solar module array is divided into multiple branches and connected to the controller through a junction box. When the battery is fully charged, the controller will disconnect the component array one by one; The load is jointly powered by the battery and the remaining photovoltaic modules. When the battery voltage drops to the set value, the controller will connect the module array one by one to adjust the charging voltage and current of the battery pack. This incremental control method can approximate the effect of pulse width modulation (PWM) controller, with more channels, smaller amplification, and closer to linear regulation.

3. Application examples

An example of a photovoltaic power supply system:

The local latitude is 11 ° 59 ′ N, and the number of days of continuous rainy and overcast weather is 5. The base station load types are BTS and microwave, with a load power of 550W. Based on this information, the system configuration is as follows:
Photovoltaic modules: Monocrystalline 165W 30 modules;
Battery pack: 2 sets of 2V 1000Ah OPzV gel batteries;
Charge and discharge controller: -48V 150A controller.

The project is equipped with 22 solar powered BTS stations, with a load power consumption range of 400-900W and relatively small capacity. If diesel generators are used for power supply, the capacity of the oil engine is small, the conversion efficiency of the oil engine is reduced, resulting in poor economy and low power supply reliability. Adopting a solar cell and gel sealed battery power supply scheme, there is no need to regularly refuel the fuel tank or maintain the diesel generator and rich liquid battery. It saves diesel purchase costs, reduces maintenance workload, and effectively lowers the operating costs of operators.